Halftoning is performed to render continuous tone digital grayscale and color images into patterns of pixels that can be displayed on bi-level devices such as printers. The rendered images are commonly referred to as halftone images. The pixels are arranged in patterns such that a halftone image creates the illusion of continuous tones.
The halftone images may contain halftone patterns composed of isolated pixels. However, isolated pixels are not reliably reproduced by laser printers and other electro-photographic devices.
The halftone images may contain clusters of pixels. The pattern power spectra of the clustered pixels exhibits a strong mid-frequency component, as opposed to the strong high frequency component exhibited by the isolated pixel halftone patterns. Clustered pixels can be reliably reproduced by laser printers.
Error diffusion halftoning is a high quality method of rendering continuous tone images. The high quality is achieved by non-linear feedback. Quantization errors a refiltered using an error filter, and the filtered errors are fed back to the input in order to shape the quantization noise into high frequency regions, which are less perceptible to the human visual system. As a result, the error diffusion halftoning can produce dot patterns that are visually pleasing.
It can be advantageous to use the error diffusion halftoning to produce clustered pixel halftone patterns. Such halftoning can produce patterns that are robust to printer distortions due to the poor isolated pixel reproduction by laser printers and other electro-photographic devices.